Appended or Linked Hair Particles not like original Hair Particles
I discovered that although my character only has one rig, it is also linking/ appending a 2nd rig which is indirectly connected via animation = the boy rides a bike. The 2nd rig when deleted from the file, causes the head object to re-scale and restores the styled hair.

It would be easy to conclude the problem is with the scale of the rigs. The 1st is scaled 0.1 to fit into the environment. All other objects, including the 2nd rig, were scaled 1:1. In the initial scene this presented no problems and indeed should not. I have performed simply tests with differently armatures in different scales, added a few animations and none of them have changed the representation of the hair particles.

In this case Blender has not been able to reconcile the different scales and my solution is to sacrifice the animation - this gets destroyed when the second rig is deleted - if my priority is to get the characters hair restored. Unfortunately at this stage I cannot Apply Scale as this also causes strange results. For future reference I will of course do this before animation just to rule out the possibility that this could contribute to Blender confusion.

Regarding the disappearance of my Vertex Groups in answer to Dimali's suggested answer and my comment: During the (pitchipoy) rigging process I had to re do the bone placements a number of times and each time the cleanest way to reset the weights was to delete all groups and start again. My hair particles being hidden at the time survived the purge, but the groups did not. While not a documented function, it appears similar to applying a modifier group and not being able to return to the edit stage. As I do not need to edit the groups I do not consider this a problem.